Meet the TransistorJanuary
1955 Popular Electronics

Wax nostalgic about and learn from the history of early electronics. See articles
from Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.

Shortly
before Christmas, 1947, the experimental work of Bell Laboratories
scientists John Bardeen, Walter Brattain, and William Shockley resulted
in the world's first semiconductor transistor. With proper biasing,
the germanium transistor demonstrated an ability to produce signal
gain. The signal fed to the base resulted in a higher amplitude
signal at the collector. Voila, the device which would ultimately
replace the vacuum electron tube had been invented. The rest, as
they say, is history. Aside from a few high power applications,
the only new equipment produced that uses vacuum tubes are retro
things like audio amplifiers and simple receivers. Of course, there
is still a large cadre of vacuum tube users in the Amateur Radio
real and vintage equipment restorers. If you have never watched
a chassis full of tubes turn on and begin glowing, it is worth your
while to find someone with an old radio - or even a TV - and take
in the nostalgia.

A tiny component that will revolutionize the entire electronics
industry - can operate from dry cells.

Few electronic
inventions have captured the public's interest as has the development
of the transistor. This device, although requiring but 1/1000th
the power and a small fraction of the space of a vacuum tube, may
be used as an amplifier, detector, or oscillator and is thus capable
of handling many of the jobs that vacuum tubes are normally employed
to do. In addition, since the transistor has no filament to burn
out and because it operates at comparatively low temperatures, it
has a life expectancy from 10 to 20 times greater than the average
tube.

Fig. 1. Miniature vacuum lube dwarfs the transistor which can
replace it. Later models are even tinier.

Fig. 2. A cutaway view of a point-contact transistor. Note two
"cat's whiskers".

For these reasons and because of its small size (see Fig. 1) it
has virtually replaced tubes in the manufacture of hearing aids.
It is also being used in military equipment, specialized communications
gear, and in certain types of instruments. Eventually it will be
used in portable receivers, home radio and TV sets, and auto receivers.
Tiny transistorized "wristwatch" and pocket-sized transmitters and
receivers have already been built experimentally and offer commercial
possibilities.

Transistors are made possible by the electrical
properties of a group of materials known as semi-conductors, consisting
of substances which may act either as conductors or insulators,
depending on their physical conditions. Germanium, silicon, and
selenium are the most popular semi-conductors, with germanium being
used almost exclusively in the manufacture of transistors.

In a normal conductor, such as copper or silver, current flows when
an electrical voltage is applied to the material causing a movement
of free electrons through the substance. In a semi conductor, the
application of voltage alone may not be sufficient to initiate current
flow - some other physical condition may be necessary such as the
presence of light, heat, or of an additional electrical field. The
current flow, when it does take place, may consist not only of the
movement of free electrons but may also include the movement of
electrical "holes" through the material.

A "hole"
is formed when an individual molecule loses an electron. The molecule
lacking an electron may pick up one from a nearby, electrically
neutral molecule, thus leaving the second molecule with a hole and
a net positive charge. In this way, the hole may travel through
the substance, jumping from molecule to molecule and producing a
current flow which acts just as if it consisted of movement of positively
charged particles.

Although current flow through a particular
substance may consist of a movement of both holes and electrons,
if the current flow is made up primarily of a movement of holes,
the material is called a "positive-carrier" or p-type semi-conductor.
If the current flow is made up primarily of a movement of electrons,
it is called a "negative-carrier" or n-type semi-conductor. A transistor
is made up of a combination of these materials.

Types of Transistors

Transistors are usually divided
into two basic types, depending on their method of construction,
i.e., the point-contact and the junction types. A cutaway view of
a point-contact type is shown in Fig. 2 while a junction type is
illustrated in Fig. 4.

A point-contact transistor consists
of a small cube of semi-conductor material with two fine wires or
"cat's whiskers" contacting its surface. Electrical connections
are made to the semi-conductor, called the "base", and to each of
the two contact wires - one of which is called the "emitter", the
other the "collector". If n-type semi-conductor material is used,
it is called an n-base point-contact transistor. Small p-type areas
are formed under the contact wires during manufacture. If p-type
material is used in the base, the transistor is a p-base unit, and
small n-type areas are formed under the tips of the contacts.

A junction transistor consists of a "sandwich" of two types
of semi-conductor material, with the inner layer of different material
from the two outer layers. The basic construction is shown in Fig.
4. If the inner layer is of p-type material, the unit is called
an n-p-n junction transistor - if of n-type material, a p-n-p transistor
results. The n-p-n type is shown in Fig. 4. Electrical connections
are made to the two outer layers and to the inner layer of the "sandwich",
with terminals identified as "emitter", "base", and "collector",
just as in the case of the point-contact type.

How
Transistors Work

Operation of the transistor may
be understood by referring to a basic transistor amplifier circuit
(see Fig. 3A). An n-p-n junction transistor is used.

In
operation, the emitter-base circuit is "biased" by battery B1
in such a way that a low resistance is offered to the flow of current
through the n-p emitter-base junction. The collector-base circuit,
on the other hand, is biased with reverse polarity by battery B2
and offers a high resistance to the flow of current, in fact, current
flow can only take place through the p-n base collector junction
because of the excess of electrons produced by the current flow
in the emitter-base circuit.

If a signal is applied to the input terminals (across R1)
the variations in the emitter-base current which result will
cause a variation in the number of free electrons in the base,
with resulting changes appearing as an amplified signal across
R2. In practice, the emitter and collector currents
may be on the same order of magnitude, but a considerable signal
power gain is obtained since the collector circuit represents
a high impedance while the input (emitter-base) circuit represents
a low impedance.

Operation of a p-n-p transistor amplifier
is similar except that conduction in the base-collector circuit
is principally by means of "holes" instead of electrons.

The schematic symbol used to identify an n-p-n type transistor
in wiring diagrams is shown in Fig. 3B. The base is represented
by a straight line, with the emitter and collector terminals
identified by slanting lines to the base with the emitter further
identified by an arrowhead pointing away from the base.

The p-n-p type transistor is wired directly opposite from
the n-p-n type, that is, the collector voltage is negative while
the emitter voltage is positive. The symbol for the p-n-p type
is identical to that for the n-p-n except that the arrowhead
points toward the base instead of away from it.

In this
article we have discussed the basic principles of transistors
and explained how they can be used in a simple amplifier. Subsequent
issues will describe other amplifiers as well as detectors and
oscillator circuits using transistors. END

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